1. Field of the Invention
This invention relates to a method of supporting seamless hand-off in a mobile telecommunications network, especially to hand-offs to a roaming mobile terminal in the network.
2. Description of the Related Art
In third generation telecommunications networks such as GPRS (General Packet Radio Service) and EDGE (Enhanced Data-rate for GSM Evolution), when a mobile terminal moves into a foreign network, network connectivity is optionally maintained by the use of Mobile Internet Protocol (Mobile IP). In the home network, a Home Agent (HA) is set up which maintains the location information of the mobile by use of Binding Updates, i.e., registration of information sent to the HA by the mobile node.
Mobile IP has two working modes. The first is illustrated in FIG. 1; a mobile terminal is currently attached as a Mobile Node (MN) 10 in a network different from its home network. The MN 10 is communicating with a Correspondent Node (CN) 12. A Home Agent 14 is set up in the home network by the CN 12, and a Foreign Agent (FA) 16 is set up in the foreign network. The FA 16 allocates a unique IP address for the visiting mobile, a Care of Address (COA) and this address is sent to the HA 14 in a Binding Update.
Packets for the mobile are encapsulated by the HA 14 and tunneled along tunnel 18 to the FA 16 for transmission to MN 10. In such encapsulation, an extra IP header is added to each packet, including the COA of the MN 10. This is known as FA-COA working mode.
In the second working mode (not illustrated) there is no FA, the MN 10 is allocated a unique COA and encapsulated packets are tunneled by HA 14 directly to MN 10; this is known as Collocated Care of Address mode of working (CO-COA).
In a conventional packet switched network, when a host A sends a packet to another host B, host A needs to determine the Media Access Control (MAC) address of host B so that the packet can be delivered to the correct physical address in layer 2. Host A sends a MAC broadcast frame, called an Address Resolution Protocol (ARP) request frame, which contains the host A's IP and MAC addresses and the IP address of host B. All nodes in the local network receive the broadcast IP request frame, and compare the destination address with their own IP address. Only the node with the correct IP address of the ARP request responds by sending a ARP reply containing its MAC address. On receiving the ARP reply, host A updates its ARP cache, which usually times out periodically. After an ARP cache entry has timed out for a specific host, the ARP request is sent again to discover the MAC address of the destination. The ARP cache is consulted by a host before it sends an ARP request, and if the answer is found in the cache, the host does not need to generate an ARP request.
It is known to provide a proxy ARP which is an ARP reply sent by one node on behalf of another node which is unwilling or unable to answer its own ARP requests. The sender of a proxy ARP reverses the sender and target protocol address fields and supplies some configured MAC address (generally its own) in the sender hardware address field (the place-holder field). The node receiving the reply then associates this link-layer address with the IP address of the original target node, so that future packets for this target node are transmitted to that MAC address.
When a mobile roams it maintains connectivity by the use of a unique IP address allocated to it, a Care-of-Address, as explained above. By use of Mobile IP, a packet sent from a correspondent node to a mobile node can still use the home address of the mobile node no matter where the mobile moves to. The packet received in the foreign network needs to bear the current Care of Address of the mobile as the destination address. If applications running on the mobile node still use the home address of the mobile, there is seamless mobility support to those applications; the applications do not have to stop and re-start as the mobile roams.
However, before the packet can be sent from the last routing switch in the foreign network to the current Care of Address from the mobile, the routing switch needs to know the MAC address of the mobile. In the current functional specifications of ARP and of Mobile IP, this entry cannot be created. Referring again to FIG. 1, in the terms used above, either CN 12 or MN 10 can be host A, the other being host B. In either direction, packets passed through the network from router to router, and in FIG. 1 two routers 22, 24, are shown schematically adjacent the optimized route 20. Suppose packets are passing from CN 12 to FA 16 via routers 22, 24; before a packet can be sent from router 24 to the current Care of Address of MN 10, router 24 needs to know the MAC address of MN 10, as stated above.
The routers 22, 24 can alternatively be associated with the encapsulated route 18.